Zoom lens system

ABSTRACT

A zoom lens system for use with compact cameras which is compact and yet effectively corrects chromatic aberrations, includes, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length, the inter-group distance being varied to perform zooming. The first lens group includes, in order from the object side, a first subgroup having a small power and a second subgroup having a large positive power. At least one of the subgroups is a cemented lens having a divergent surface, with an aspheric surface being provided in the first subgroup. The first lens group satisfies the following conditions 
     
         -0.7&lt;fS/fla&lt;0.3 
    
     
         1.2&lt;fS/flb&lt;2.3 
    
     
         -1.7&lt;fS/fc&lt;0 
    
     where: 
     fS=the overall focal length at wide angle, 
     fla=the focal length of the first subgroup, 
     flb=the focal length of the second subgroup.

BACKGROUND OF THE INVENTION

The present invention relates to a small zoom lens system for use withcompact cameras that has a smaller constraint on its back focus than azoom lens for use with single-lens reflex cameras.

Heretofore, a class of two-group zoom lens systems that do not employaspheric surfaces have been used with compact cameras. An example ofthese types of systems is one in which the first lens group is composedof several elements, and the second lens group is composed of aboutthree elements. Thus, the exemplary system has about eight elements (seecommonly-assigned Japanese Patent Laid-Open Publication No. SHO62-264019).

Another class of conventional zoom lens systems for use with compactcameras includes one that uses numerous aspheric surfaces and in whichthe first and second lens groups are each composed of two elements,thereby resulting in a system having four elements total (seecommonly-assigned Japanese Patent Laid-Open Publication No.HEI-3-127008), as well as one which is composed of fewer lens elements,such as two or three in total number.

However, the conventional zoom lens systems for use with compact camerashave had various problems. Two-group systems that do not use asphericsurfaces need so many lens elements that making the system compact isdifficult.

With four-element systems that have numerous aspheric surfaces, it isdifficult to attain balance between reduction in the overall lens lengthand correction of chromatic aberrations.

Two- or three-element systems have a generally poor performance.Furthermore, it has been difficult to attain balance between reductionin the overall lens length and correction of chromatic aberrations.

Therefore, an object of the present invention is to provide a zoom lenssystem for use with compact cameras which is compact (i.e., hasrelatively fewer lens elements) and yet which effectively correctschromatic aberrations.

SUMMARY OF THE INVENTION

To attain the above-described object, the zoom lens system of thepresent invention comprises, in order from the object side, a first lensgroup having a positive focal length and a second lens group having anegative focal length, the distance between the first and second lensgroups being varied to perform zooming. The first lens group comprises,in order from the object side, a subgroup 1a having a small power and asubgroup 1b having a large positive power, at least one of the subgroups1a and 1b being a cemented lens having a divergent cemented surface, andat least one lens surface of the subgroup 1a being aspheric. The firstlens group satisfies the following conditions (al), (bl) and (c):

    -0.7<fS/fla<0.3                                            (a1)

    1.2<fS/flb<2.3                                             (b1)

    -1.7<fS/fc<0                                               (c)

where

fS: the focal length of the overall system at the wide-angle end;

fla: the focal length of subgroup 1a;

flb: the focal length of subgroup 1b;

fc: the focal length of the cemented surface in the first lens group andfc=rc/(Nr-Nf); and

where

rc: the radius of curvature of the cemented surface;

Nr: the refractive index at the d-line of the lens element on the imageside of the cemented surface; and

Nf: refractive index at the d-line of the lens element on the objectside of the cemented surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of the zoom lens systemaccording to Example 1 at the wide-angle end;

FIG. 2 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 1 at the wide-angle end;

FIG. 3 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 1 at the middle-angle end;

FIG. 4 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 1 at the narrow-angle end;

FIG. 5 is a simplified cross-sectional view of the zoom lens systemaccording to Example 2 at the wide-angle end;

FIG. 6 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 2 at the wide-angle end;

FIG. 7 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 2 at the middle-angle end;

FIG. 8 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 2 at the narrow-angle end;

FIG. 9 is a simplified cross-sectional view of the zoom lens systemaccording to Example 3 at the wide-angle end;

FIG. 10 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 3 at the wide-angle end;

FIG. 11 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 3 at the middle-angle end;

FIG. 12 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 3 at the narrow-angle end;

FIG. 13 is a simplified cross-sectional view of the zoom lens systemaccording to Example 4 at the wide-angle end;

FIG. 14 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 4 at the wide-angle end;

FIG. 15 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 4 at the middle-angle end;

FIG. 16 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 4 at the narrow-angle end;

FIG. 17 is a simplified cross-sectional view of the zoom lens systemaccording to Example 5 at the wide-angle end;

FIG. 18 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 5 at the wide-angle end;

FIG. 19 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 5 at the middle-angle end;

FIG. 20 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 5 at the narrow-angle end;

FIG. 21 is a simplified cross-sectional view of the zoom lens systemaccording to Example 6 at the wide-angle end;

FIG. 22 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 6 at the wide-angle end;

FIG. 23 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 6 at the middle-angle end;

FIG. 24 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 6 at the narrow-angle end;

FIG. 25 is a simplified cross-sectional view of the zoom lens systemaccording to Example 7 at the wide-angle end;

FIG. 26 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 7 at the wide-angle end;

FIG. 27 illustrates set of graphs plotting the aberration curvesobtained with the lens system of Example 7 at the middle-angle end;

FIG. 28 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 7 at the narrow-angle end;

FIG. 29 is a simplified cross-sectional view of the zoom lens systemaccording to Example 8 at the wide-angle end;

FIG. 30 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 8 at the wide-angle end;

FIG. 31 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 8 at the middle-angle end; and

FIG. 32 illustrates a set of graphs plotting the aberration curvesobtained with the lens system of Example 8 at the narrow-angle end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By using aspheric lenses and cemented lenses, the present inventionsuccessfully provides zoom lens systems for use with compact camerasthat are compact and have fewer lens elements, as compared to theconventional systems, and yet which effectively correct chromaticaberrations. Compared to a first lens group being composed of twoelements in two units, the zoom lens system of the present invention hasa smaller size and greater efficiency in correcting chromaticaberrations.

By providing the cemented surface or surfaces with a divergent negativepower, the dependency on aspheric surfaces can be reduced, therebymaking it easy to manufacture aspheric lenses.

Condition (a1) relates to the power of subgroup 1a having a small power.If the upper limit of this condition is met or exceeded, the positivepower of subgroup 1a will increase and the back focus will become soshort as to make correcting distortion difficult. If the lower limit ofcondition (a1) is not exceeded, the negative power of subgroup 1a willincrease. However, the first lens group taken as a whole requires apositive power, so the positive power of subgroup 1b must beconsiderably increased if subgroup 1a has a large negative power. Thismakes correcting spherical aberration difficult.

Condition (b1) relates to the power of subgroup 1b which functions as amaster lens. If the upper limit of this condition is met or exceeded,the power of subgroup 1b will increase, causing greater variations inspherical aberration. If the lower limit of condition (b1) is notexceeded, the result is advantageous for correcting aberrations, but theamount of movement required for zooming will increase.

Condition (c) relates to the power of the cemented surface and it mustbe satisfied to efficiently correct chromatic aberrations and to reducethe overall size of the lens system. If the upper limit of thiscondition is met or exceeded, overcorrection will occur to producespherical aberrations of higher orders. If the lower limit of condition(c) is not exceeded, chromatic aberrations will be undercorrected.

If subgroup 1b has a cemented lens, the following condition (a2), (b2)and (d) are preferably satisfied:

    -0.7<fS/f1<0                                               (a2)

    1.2<fS/f2-3<2.3                                            (b2)

    -1.7<fS/fcb<-0.3                                           (d)

f2-3: the focal length of subgroup 1b;

fcb: the focal length of the cemented surface in subgroup 1b andfcb=rcb/(N3-N2); and

where

rcb: the radius of curvature of the cemented surface;

N2: the refractive index of the second lens element at the d-line; and

N3: the refractive index of the third lens element at the d-line.

If subgroup 1a has a cemented lens, the following conditions (a3), (b3)and (e) are preferably satisfied:

    -0.3<fS/f1-2<0.3                                           (a3)

    1.2<fS/f3<2.3                                              (b3)

    -0.7<fS/fca<0                                              (e)

where

f1-2: the focal length of subgroup 1a;

f3: the focal length of subgroup 1b;

fca: the focal length of the cemented surface in subgroup 1a andfca=rca/(N2-N1); and

where

rca: the radius of curvature of the cemented surface; and

N1: the refractive index of the first lens element at the d-line.

If subgroups 1a and 1b both have a cemented lens, the followingconditions (a4), (b4) and (f) are preferably satisfied:

    -0.3<fS/f1-2<0.3                                           (a4)

    1.2<fS/f3-4<2.3                                            (b4)

    -1.7<fS/fcb<-0.3                                           (f)

where

f1-f2: the focal length of subgroup 1a;

f3-4: the focal length of subgroup 1b;

fcb: the focal length of the cemented surface in subgroup 1b andfcb=rcb/(N4-N3); and

where

N4: the refractive index of the fourth lens element at the d-line.

The combinations of conditions (a2), (b2) and (d), conditions (a3), (b3)and (e), and conditions (a4), (b4) and (f) will perform the samefunctions as the combination of conditions (a1), (b1) and (c).

When subgroups 1a and 1b both have a cemented lens, an aspheric surfacehaving a large negative divergent power is preferably provided in thecemented lens forming subgroup 1b having a large positive power.

To ensure that the first lens group will have an increased power whilereducing the number of constituent lens elements sufficiently to achievethe desired size reduction, the lens surface closest to the object ispreferably made concave.

This is particularly advantageous when the first lens group is designedas a retrofocus type of short overall length to assure a wider viewangle at the short-focus end.

The first lens of the first lens group preferably satisfies thefollowing condition (g) which specifies the degree of curvature of thesurface of the surface that faces the object:

    -15<SF1<-0.3                                               (g)

where

SF1: the shape factor of the first lens element of the first lens groupand SF1=(r1+r2)/(r1-r2); and

where

r1: the radius of curvature of the first surface of the first lenselement; and

r2: the radius of curvature of the second surface of the first lenselement.

If the lower limit of this condition is not exceeded, the concavesurface facing the object will have an excessive radius of curvature,leading to the development of higher-order aberrations. If the upperlimit of condition (g) is met or exceeded, the power of the first lensgroup will not be able to be increased while keeping the overall sizesufficiently small because it is difficult to correct aberrations or,alternatively, more constituent lens elements have to be used.

When subgroup 1a is provided with an aspheric surface, the zoom lenssystem of the present invention preferably satisfies the followingadditional condition (h) concerning the optical glass material to formthe aspheric lens in subgroup 1a:

    34<νa                                                   (h)

where

νa: the Abbe number of the first lens element of the first lens group.

If a glass molded aspheric lens is to be used as the first subgroup, itis difficult to manufacture it from an SF-type optical glass material.Hence, a glass material is desirably used which does not satisfycondition (h).

In the present invention, a cemented lens is used in either subgroup 1aor 1b or both, and which accomplishes achromatism easily without usingan SF-type optical glass material in the aspheric lens.

When a cemented lens is used in subgroup 1b, the optical glass materialfor forming the negative lens element in subgroup 1b preferablysatisfies the following condition (i):

    νb<34                                                   (i)

where

νb: the Abbe number of subgroup 1b (cemented lens).

By making the negative lens element in subgroup 1b of an SF type opticalglass material that satisfies condition (i), desired achromatism can beaccomplished by subgroup 1b without using an SF-type optical glassmaterial in subgroup 1a which is provided with an aspheric surface.

In another preferred embodiment, the zoom lens system of the presentinvention may satisfy the following additional condition (j), whichconcerns the aspheric surface in subgroup 1a:

    0<ΔV<2                                               (j)

where

ΔV: the sum of variations in the coefficient of the third-orderdistortion due to the aspheric surface in subgroup 1a.

Generally, the shorter the overall length of the system, the more likelythat a positive distortion will develop on the shorter-focus side.

Hence, if the upper limit of condition (j) is met or exceeded,over-correction will occur, increasing the likelihood of a greaterdistortion at a middle-angle end than at a maximum-angle end. If thelower limit of condition (j) is not exceeded, distortion will beinsufficiently corrected.

Various modifications can be made without departing from the spirit ofthe present invention and they include, for example, a system in whichsubgroups 1a and 1b are movable independently of each other, as well asa system in which a lens group having a small power is provided behindthe second lens group.

The following is a supplemental discussion of the amount of variation inthe coefficient of the third-order aberration due to an asphericsurface. The shape of an aspheric surface can generally be expressed byequation (1): ##EQU1## where x is the distance by which the coordinatesat the point on the aspheric surface where the height from the opticalaxis is y, departs from the plane tangent to the vertex of the asphericsurface; c is the curvature (1/r) of the vertex of the aspheric surface;K is the conic constant; and α4, α6, α8 and α10 are the asphericcoefficients of the fourth, sixth, eighth and tenth orders,respectively.

By calculating the focal length as f=1.0, namely, by substituting x=x/f,y=y/f, c=fc, A4=f³ α4, A6=f⁵ α6, A8=f⁷ α8 and A10=f⁹ α10 into equation(1), the following equation (2) is obtained: ##EQU2##

The second and subsequent terms of equation (2) define the amount ofsphericity and the relationship between the coefficient A4 of the secondterm and the third-order aspheric coefficient φ is expressed by:

    φ=8(N'-N)A4

where N is the refractive index of the medium before the asphericsurface and N' is the refractive index of the medium after the asphericsurface. The aspheric coefficient φ provides the following amounts ofvariation in third-order aberration coefficients as referred to in thetheory of aberrations:

    ΔI=h.sup.4 φ

    ΔII=h.sup.3 Hφ

    ΔIII=h.sup.2 H.sup.2 φ

    ΔIV=h.sup.2 H.sup.2 φ

    ΔV=hH.sup.3 φ

where

I: a spherical aberration coefficient;

II: a coma coefficient;

III: an astigmatism coefficient;

IV: a sagittal field curvature coefficient;

V: a distortion coefficient;

h: the height of paraxial on-axis rays at which they pass through eachlens surface; and

H: the height of paraxial off-axis rays passing through the center ofthe pupil, at which they pass through each lens surface.

When the aspheric surface in subgroup 1a satisfies condition (j), thezoom lens system of the present invention preferably satisfies thefollowing additional condition (k), which concerns the thickness of thefirst lens element:

    0.1<d1/fS<0.3                                              (K)

where

d1: the thickness of the first lens element of the first lens group.

Condition (k) should particularly be satisfied in order to provide awider view angle. If the lower limit of this condition is not exceeded,difficulty is involved in providing a wider view angle. If the upperlimit of condition (k) is met or exceeded, the thickness of the firstlens will increase so much as to make it difficult to produce a lighterzoom lens system.

Examples 1 to 8 of the zoom lens system of the present invention aredescribed below with reference to data sheets, in which f denotes thefocal length, fB is the back focus, r is the radius of curvature of anindividual lens surface, d is the lens thickness or the air-spacebetween lenses (the foregoing parameters are in millimeters), FNO is theF number, ω is the half view angle (in degrees), n is the refractiveindex of an individual lens at the d-line, and ν is the Abbe number ofan individual lens at the d-line, with the numbers for aspheric surfacesbeing marked with an asterisk. For each aspheric surface, α4, α6 and α8(and α10, if any) denotes the aspheric coefficients of the fourth, sixthand eighth orders (and tenth orders, if any), respectively.

Example 1

FIG. 1 is a simplified cross-sectional view of the zoom lens systemaccording to Example 1 at the wide-angle end. Specific data for thisexample are as shown in Table 1. FIG. 2 shows a set of graphs plottingthe curves of spherical aberration SA, sine condition SC, chromaticaberrations expressed by spherical aberration at the d-, g- and C-lines, lateral chromatic aberration, astigmatism (S, sagittal; M,meridional) and distortion, that are obtained with the lens system ofExample 1 at the wide-angle end; FIG. 3 shows a set of graphs plottingthe curves of the same parameters that are obtained at the middle-angleend; and FIG. 4 shows a set of graphs plotting the curves of the sameparameters that are obtained at the narrow-angle end.

                  TABLE 1                                                         ______________________________________                                        Surface                                                                       No.       r        d           n     ν                                     ______________________________________                                         1*       -16.621  5.00        1.73077                                                                             40.5                                     2         -22.148  0.10                                                       3         24.706   3.54        1.53172                                                                             48.9                                     4         -5.355   2.84        1.80518                                                                             25.4                                     5         -9.087   Variable                                                    6*       -66.142  2.10        1.58547                                                                             29.9                                     7         -17.650  3.05                                                       8         -7.160   1.80        1.83481                                                                             42.7                                     9         -52.228                                                             ______________________________________                                        First surface: aspheric                                                                        Sixth surface: aspheric                                      K = 0            K = 0                                                        α.sub.4 = -0.15769029 × 10.sup.-3                                                  α.sub.4 = 0.17904943 × 10.sup.-3                 α.sub.6 = 0.30802121 × 10.sup.-5                                                   α.sub.6 = 0.63433168 × 10.sup.-6                 α.sub.8 = -0.32510966 × 10.sup.-7                                                  α.sub.8 = 0.40777362 × 10.sup.-7                 ______________________________________                                    

The values of FNo., f, fB, ω and d5 vary with zooming, as shown in Table2 below.

                  TABLE 2                                                         ______________________________________                                        FNo.     4.7            5.7    7.9                                            ______________________________________                                        f        29.00          35.00  48.50                                          fB       12.11          17.71  30.31                                          ω  36.0           31.4   24.0                                           d5       6.03           4.67   2.84                                           ______________________________________                                    

Example 2

FIG. 5 is a simplified cross-sectional view of the zoom lens systemaccording to Example 2 at the wide-angle end. Specific data for thisexample are as shown in Table 3. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 6, 7 and 8, respectively.

                  TABLE 3                                                         ______________________________________                                        Surface                                                                       No.       r        d           n     ν                                     ______________________________________                                         1*       -16.014  5.69        1.58913                                                                             61.2                                     2         -22.672  0.10                                                       3         19.720   3.26        1.53172                                                                             48.9                                     4         -5.724   3.00        1.80518                                                                             25.4                                     5         -9.926   Variable                                                    6*       -45.870  2.23        1.58547                                                                             29.9                                     7         -15.519  2.95                                                       8         -7.032   1.50        1.83481                                                                             42.7                                     9         -44.700                                                             ______________________________________                                        First surface: aspheric                                                                        Sixth surface: aspheric                                      K = 0            K = 0                                                        α.sub.4 = -0.12476019 × 10.sup.-3                                                  α.sub.4 = 0.16558446 × 10.sup.-3                 α.sub.6 = 0.26422229 × 10.sup.-5                                                   α.sub.6 = 0.12079021 × 10.sup.-5                 α.sub.8 = -0.23612453 × 10.sup.-7                                                  α.sub.8 = 0.37475794 × 10.sup.-7                 ______________________________________                                    

The values of FNo., f, fB, ω and d5 vary with zooming, as shown in Table4 below.

                  TABLE 4                                                         ______________________________________                                        FNo.     4.7            5.7    7.9                                            ______________________________________                                        f        29.00          35.00  48.50                                          fB       12.16          17.78  30.45                                          ω  36.0           31.5   24.1                                           d5       6.03           4.64   2.77                                           ______________________________________                                    

Example 3

FIG. 9 is a simplified cross-sectional view of the zoom lens systemaccording to Example 3 at the wide-angle end. Specific data for thisexample are as shown in Table 5. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 10, 11 and 12, respectively.

                  TABLE 5                                                         ______________________________________                                        Surface                                                                       No.       r         d          n     ν                                     ______________________________________                                         1*       27.076    1.70       1.80518                                                                             25.4                                      2*       17.353    2.54                                                      3         23.110    3.50       1.51633                                                                             64.1                                     4         -10.000   1.50       1.62004                                                                             36.3                                      5*       -12.606   Variable                                                   6*       -16.804   2.50       1.58547                                                                             29.9                                     7         -12.012   4.30                                                      8         -10.000   1.80       1.77250                                                                             49.6                                     9         -129.785                                                            ______________________________________                                        First surface: Aspheric                                                                         Second surface: Aspheric                                    K = 0             K = 0                                                       α.sub.4 = -0.78071698 × 10.sup.-3                                                   α.sub.4 = -0.8932763 × 10.sup.-3                α.sub.6 = -0.38820565 × 10.sup.-5                                                   α.sub.6 = -0.15051096 × 10.sup.-5               α.sub.8 = 0.42306964 × 10.sup.-7                                                    α.sub.8 = 0.73608431 × 10.sup.-7                Fifth surface: Aspheric                                                                         Sixth surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = 0.11275623 × 10.sup.-3                                                    α.sub.4 =  -0.29921502 × 10.sup.-4              α.sub.6 = 0.25817645 × 10.sup.-5                                                    α.sub.6 = 0.17753430 × 10.sup.-5                α.sub.8 = -0.22261271 × 10.sup.-7                                                   α.sub.8 = -0.41314155 × 10.sup.-7                                 α.sub.10 = 0.49999999 × 10.sup.-9                        Ninth surface: Aspheric                                                       K = 0                                                                         α.sub.4 = -0.59862508 × 10.sup.-4                                 α.sub.6 = 0.57075458 × 10.sup.-6                                  α.sub.8 = -0.54521998 × 10.sup.-8                                 α.sub.10 = 0.19963241 × 10.sup.-10                       ______________________________________                                    

The values of FNo., f, fB, ω and d5 vary with zooming, as shown in Table6 below.

                  TABLE 6                                                         ______________________________________                                        FNo.     4.5            6.3    8.5                                            ______________________________________                                        f        35.85          50.00  68.00                                          fB       11.35          23.58  39.13                                          ω  30.5           23.0   17.5                                           d5       8.47           5.24   3.07                                           ______________________________________                                    

Example 4

FIG. 13 is a simplified cross-sectional view of the zoom lens systemaccording to Example 4 at the wide-angle end. Specific data for thisexample are as shown in Table 7. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 14, 15 and 16, respectively.

                  TABLE 7                                                         ______________________________________                                        Surface                                                                       No.       r         d          n     ν                                     ______________________________________                                         1*       63.664    1.30       1.80100                                                                             35.0                                     2         19.256    2.00       1.72916                                                                             54.7                                      3*       57.630    3.34                                                       4*       92.721    3.00       1.51728                                                                             69.6                                     5         -11.731   Variable                                                   6*       -29.116   2.50       1.58547                                                                             29.9                                     7         -19.623   4.75                                                      8         -10.000   1.80       1.77250                                                                             49. 6                                     9*       -65.660                                                             ______________________________________                                        First surface: Aspheric                                                                         Third surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = -0.37335714 × 10.sup.-3                                                   α.sub.4 = -0.50000002 × 10.sup.-3               α.sub.6 = -0.23381363 × 10.sup.-5                                                   α.sub.6 = 0.78892879 × 10.sup.-6                α.sub.8 = 0 α.sub.8 = 0                                           Fourth surface: Aspheric                                                                        Sixth surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = -0.23135254 × 10.sup.-3                                                   α.sub.4 = 0.43305326 × 10.sup.-4                α.sub.6 = 0.25132805 × 10.sup.-7                                                    α.sub.6 = 0.10069011 × 10.sup.-5                α.sub.8 = 0 α.sub.8 = 0.16350554 × 10.sup.-8                         Ninth surface: Aspheric                                                       K = 0                                                                         α.sub.4 = -0.27770270 × 10.sup.-4                                 α.sub.6 = 0.78884771 × 10.sup.-7                                  α.sub.8 = 0                                                    ______________________________________                                    

The values of FNo., f, fB, ω and d5 vary with zooming, as shown in Table8 below.

                  TABLE 8                                                         ______________________________________                                        FNo.     4.5            6.2    8.5                                            ______________________________________                                        f        36.05          50.00  68.00                                          fB       11.30          23.35  38.90                                          ω  30.5           23.1   17.5                                           d5       7.91           4.69   2.49                                           ______________________________________                                    

Example 5

FIG. 17 is a simplified cross-sectional view of the zoom lens systemaccording to Example 5 at the wide-angle end. Specific data for thisexample are as shown in Table 9. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 18, 19 and 20, respectively.

                  TABLE 9                                                         ______________________________________                                        Surface                                                                       No.       r         d          n     ν                                     ______________________________________                                         1*       -267.945  2.00       1.75520                                                                             27.5                                      2*       860.692   7.79                                                      3         32.888    3.50       1.51633                                                                             64.1                                     4         -12.150   1.50       1.80518                                                                             25.4                                      5*       -15.614   Variable                                                   6*       357.749   3.00       1.58547                                                                             29.9                                     7         -48.998   4.81                                                      8         -10.688   1.50       1.77250                                                                             49.6                                     9         -113.863                                                            ______________________________________                                        First surface: Aspheric                                                                         Second surface: Aspheric                                    K = 0             K = 0                                                       α.sub.4 = -0.16637679 × 10.sup.-3                                                   α.sub.4 = 0.13850736 × 10.sup.-3                α.sub.6 = -0.12474394 × 10.sup.-5                                                   α.sub.6 = -0.93495528 × 10.sup.-6               α.sub.8 = -0.25899414 × 10.sup.-7                                                   α.sub.8 = 0.28773396 × 10.sup.-7                Fifth surface: Aspheric                                                                         Sixth surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = 0.242538567 ×10.sup.-4                                                    α.sub.4 = 0.63100489 × 10.sup.-4                α.sub.6 = 0.64685131 × 10.sup.-7                                                    α.sub.6 = 0.14545819 × 10.sup.-6                α.sub.8 = -0.20730672 × 10.sup.-8                                                   α.sub.8 = 0.27117914 × 10.sup.-8                ______________________________________                                    

The values of FNo., f, fB, ω and d5 vary with zooming, as shown in Table10 below.

                  TABLE 10                                                        ______________________________________                                        FNo.     3.6            5.5    8.0                                            ______________________________________                                        f        39.13          60.00  87.00                                          fB       13.19          31.28  54.68                                          ω  28.4           19.6   13.9                                           d5       8.33           3.72   1.03                                           ______________________________________                                    

Example 6

FIG. 21 is a simplified cross-sectional view of the zoom lens systemaccording to Example 6 at the wide-angle end. Specific data for thisexample are as shown in Table 11. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 22, 23 and 24, respectively.

                  TABLE 11                                                        ______________________________________                                        Surface                                                                       No.       r         d          n     ν                                     ______________________________________                                         1*       223.230   1.50       1.80100                                                                             35.0                                     2         25.220    2.80       1.60311                                                                             60.7                                      3*       -188.485  6.29                                                      4         36.285    3.50       1.55963                                                                             61.2                                      5*       -19.279   Variable                                                   6*       -168.929  3.00       1.58547                                                                             29.9                                     7         -53.327   4.81                                                      8         -10.072   1.50       1.77250                                                                             49.6                                     9         -53.268                                                             ______________________________________                                        First surface: Aspheric                                                                         Third surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = -0.15064565 × 10.sup.-3                                                   α.sub.4 = -0.18686928 × 10.sup.-3               α.sub.6 = -0.11185335 × 10.sup.-5                                                   α.sub.6 = -0.11629641 × 10.sup.-5               α.sub.8 = 0.54297900 × 10.sup.-8                                                    α.sub.8 = 0.14898899 × 10.sup.-7                Fifth surface: Aspheric                                                                         Sixth surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = 0.53898116 × 10.sup.-4                                                    α.sub.4 =  0.76097797 × 10.sup.-4               α.sub.6 = 0.37592790 × 10.sup.-6                                                    α.sub.6 = 0.21590713 × 10.sup.-4                α.sub.8 = -0.75734043 × 10.sup.-8                                                   α.sub.8 = 0.39379218 × 10.sup.-8                ______________________________________                                    

The values of FNo., f, fB, ω and d5 vary with zooming, as shown in Table12 below.

                  TABLE 12                                                        ______________________________________                                        FNo.     3.6            5.5    8.0                                            ______________________________________                                        f        39.16          60.00  87.00                                          fB       12.63          29.87  52.20                                          ω  28.3           19.6   13.9                                           d5       7.80           3.83   1.51                                           ______________________________________                                    

Example 7

FIG. 25 is a simplified cross-sectional view of the zoom lens systemaccording to Example 7 at the wide-angle end. Specific data for thisexample are as shown in Table 13. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 26, 27 and 28, respectively.

                  TABLE 13                                                        ______________________________________                                        Surface                                                                       No.       r         d          n     ν                                     ______________________________________                                         1*       -26.434   2.00       1.74950                                                                             35.3                                      2*       -34.072   4.88                                                       3*       44.439    3.50       1.51633                                                                             64.1                                     4         -10.041   1.50       1.80518                                                                             25.4                                     5         -13.138   Variable                                                   6*       1280.856  3.00       1.58547                                                                             29.9                                     7         -44.442   4.74                                                      8         -10.845   1.50       1.77250                                                                             49.6                                     9         -117.602                                                            ______________________________________                                        First surface: Aspheric                                                                         Second surface: Aspheric                                    K = 0             K = 0                                                       α.sub.4 = -0.33992982 ×10.sup.-3                                                    α.sub.4 = -0.30307845 ×10.sup.-3                α.sub.6 = 0.11890545 × 10.sup.-5                                                    α.sub.6 = 0.29612906 × 10.sup.-5                α.sub.8 = 0.47529293 × 10.sup.-7                                                    α.sub.8 = 0.13834532 × 10.sup.-7                α.sub.10 = -0.25598455 × 10.sup.-9                                Third surface: Aspheric                                                                         Sixth surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = -0.96392728 × 10.sup.-4                                                   α.sub.4 = 0.63255518 × 10.sup.-4                α.sub.6 = 0.10985129 × 10.sup.-5                                                    α.sub.6 = 0.11434083 × 10.sup.-7                α.sub.8 = -0.60582298 × 10.sup.-8                                                   α.sub.8 = -0.40663572 × 10.sup.-8               ______________________________________                                    

The values of FN6., f, fB, ω and d5 vary with zooming, as shown in Table14 below.

                  TABLE 14                                                        ______________________________________                                        FNo.     3.6            5.5    8.0                                            ______________________________________                                        f        39.13          60.00  87.00                                          fB       14.13          32.90  57.19                                          ω  28.4           19.7   13.9                                           d5       8.72           4.15   1.49                                           ______________________________________                                    

Example 8

FIG. 29 is a simplified cross-sectional view of the zoom lens systemaccording to Example 8 at the wide-angle end. Specific data for thisexample are as shown in Table 15. The aberration curves obtained withthis lens system at the wide-angle, middle-angle and narrow-angle endsare plotted in FIGS. 30, 31 and 32, respectively.

                  TABLE 15                                                        ______________________________________                                        Surface                                                                       No.       r         d          n     ν                                     ______________________________________                                         1*       -69.265   1.50       1.80100                                                                             35.0                                     2         108.973   2.80       1.60311                                                                             60.7                                      3*       -43.268   6.49                                                       4*       38.453    4.00       1.55963                                                                             61.2                                     5         -9.000    1.50       1.83481                                                                             42.7                                     6         -14.802   Variable                                                   7*       -105.868  3.15       1.58547                                                                             29.9                                     8         -26.948   4.69                                                      9         -10.612   1.50       1.77250                                                                             49.6                                     10*       1012.067                                                            ______________________________________                                        First surface: Aspheric                                                                         Third surface: Aspheric                                     K = 0             K = 0                                                       α.sub.4 = -0.14422600 × 10.sup.-3                                                   α.sub.4 = -0.16767500 × 10.sup.-3               α.sub.6 = -0.38866300 × 10.sup.-6                                                   α.sub.6 = 0                                           α.sub.8 = 0 α.sub.8 = 0.50306800 × 10.sup.-8                α.sub.10 = 0.57327400 × 10.sup.-10                                Fourth surface: Aspheric                                                                        Seventh surface: Aspheric                                   K = 0             K = 0                                                       α.sub.4 = -0.22843900 × 10.sup. -4                                                  α.sub.4 = 0.24516400 × 10.sup.-4                α.sub.6 = -0.20316900 × 10.sup.-6                                                   α.sub.6 = 0.86419200 × 10.sup.-6                α.sub.8 = 0.10334300 × 10.sup.-7                                                    α.sub.8 = -0.15337400 × 10.sup.-7                                 α.sub.10 = 0.14726400 × 10.sup.-9                        Tenth surface: Aspheric                                                       K = 0                                                                         α.sub.4 = -0.35996000 × 10.sup.-4                                 α.sub.6 = 0.31727400 × 10.sup.-6                                  α.sub.8 = -0.29930800 × 10.sup.-8                                 α.sub.10 = 0.10557300 × 10.sup.-10                       ______________________________________                                    

The values of FNo., f, fB, ω and d6 vary with zooming, as shown in Table16 below.

                  TABLE 16                                                        ______________________________________                                        FNo.     3.6            5.9    8.0                                            ______________________________________                                        f        39.18          65.00  87.00                                          fB       12.81          33.57  51.26                                          ω  28.2           18.2   13.9                                           d5       7.90           3.29   1.52                                           ______________________________________                                    

Table 17 shows values that satisfy the conditions (a1) to (k) inExamples 1-8.

                  TABLE 17-1                                                      ______________________________________                                        Condition  Ex. 1   Ex. 2      Ex. 3 Ex. 4                                     ______________________________________                                        fS/fa      -0.20   -0.21      -0.55 -0.13                                     fS/fb      1.77    1.76       2.10  1.77                                      fS/fcb     -1.48   -1.39      -0.37 -0.14                                     SF1        -7.0    -5.8       4.6   20.1                                      νa      40.5    61.2       25.4  35.0                                      νb      25.4    25.4       --    --                                        ΔV   0.18    0.15       0.78  0.60                                      d1/fs      0.17    0.20       0.05  0.09                                      ______________________________________                                    

                  TABLE 17-2                                                      ______________________________________                                        Condition  Ex. 5   Ex. 6      Ex. 7 Ex. 8                                     ______________________________________                                        fS/fa      -0.14   -0.04      -0.22 0.04                                      fS/fb      1.67    1.70       1.33  1.57                                      fS/fcb     -0.93   -0.31      -1.13 -1.20**                                   SF1        -0.53   0.08       -7.9  4.32                                      νa      27.5    35.0       35.3  35.0                                      νb      --      --         25.4  42.7                                      ΔV   0.68    0.83       0.45  0.88                                      d1/fS      0.05    0.11       0.05  0.11                                      ______________________________________                                         **(fS/fca = -0.071)                                                      

As described above, the zoom lens system of the present invention is atwo-group type including first and second lens groups and the first lensgroup comprises, in order from the object side, a subgroup 1a having asmall power and a subgroup 1b having a large positive power, with atleast one of the subgroups 1a and 1b employing a cemented lens whereasat least one lens surface in subgroup 1a being aspheric. Because of thisarrangement, the zoom lens system of the present invention is compactand has fewer constituent lens elements as compared to the conventionalsystems and yet it efficiently corrects chromatic aberrations.

Although the present invention has been fully described by way of theexamples thereof with reference to the accompanying drawings, variouschanges and modifications will be apparent to those having skill in thisfield without departing from the scope of the invention as defined inthe appended claims.

What is claimed is:
 1. A zoom lens system, comprising:in order from thesubject side, a first lens group having a positive power; and a secondlens group having a negative power, the distance between said first lensgroup and said second lens group being varied to perform zooming, saidfirst lens group consists of, in order from the object side, a firstsubgroup and a second subgroup having a positive power, wherein saidfirst subgroup consists of a single lens element having at least oneaspheric surface, said second subgroup consists of a cemented lenshaving a divergent cemented surface, and said first lens groupsatisfying the following conditions:

    -0.7<fS/fla<0.3

    1.2<fS/flb<2.3

    -1.7<fS/fc<0

where fS: the focal length of the overall system at the wide-angle end;fla: the focal length of the first subgroup; flb: the focal length ofthe second subgroup; fc: the focal length of the cemented surface in thefirst lens group and fc=rc/(Nr-Nf); andwhere rc: the radius of curvatureof the cemented surface; and Nr: the refractive index at the d-line ofthe lens element on the image side of the cemented surface; and Nf: therefractive index at the d-line of the lens element on the object side ofthe cemented surface.
 2. A zoom lens system according to claim 1,wherein said second subgroup includes, in order from the object side, apositive second lens and a negative third lens element cementedtogether, said first lens group satisfying the following conditions:

    -0.7<fS/fla<0

    -1.7<fS/fcb<-0.3

where fcb: the focal length of the cemented surface in the secondsubgroup and fcb=rcb/(N3-N2); andwhere rcb: the radius of curvature ofthe cemented surface of the second subgroup; N2: the refractive index ofthe second lens element at the d-line; and N3: the refractive index ofthe third lens at the d-line.
 3. A zoom lens system according to claim2, wherein the negative lens element of said cemented lens comprises anaspheric lens and said cemented lens satisfies the following condition:

    νb<34

where νb: the Abbe number of the negative lens element of the secondsubgroup.
 4. A zoom lens system as recited in claim 1 furthercomprising:the single lens element of said first lens group satisfyingthe following condition:

    -15<SF1<-0.3

where SF1: the shape factor of the single lens element of the first lensgroup and SF1=(r1+r2)/(r1-r2); andwhere r1: the radius of curvature ofthe first surface of the single lens element; and r2: the radius ofcurvature of the second surface of the single lens element.
 5. A zoomlens system according to claim 1, wherein the aspheric surface in saidfirst subgroup satisfies the following condition:

    34<νa

where νa: the Abbe number of the lens element, having the asphericalsurface, of the first subgroup.
 6. A zoom lens system according to claim1, wherein the aspheric surface in the first subgroup of the first lensgroup satisfies the following condition:

    0<ΔV<2

where ΔV: the sum of variations in the coefficient of the third-orderdistortion due to the aspheric surface in the first subgroup.
 7. A zoomlens system according to claim 6, which satisfies the followingcondition:

    0.1<d1/fS<0.3

where d1: the thickness of the single lens element of the first lensgroup.
 8. A zoom lens system, comprising:in order from the subject side,a first lens group having a positive power; and a second lens grouphaving a negative power, the distance between said first lens group andsaid second lens group being varied to perform zooming, said first lensgroup consists of, in order from the object side, a first subgroup and asecond subgroup having a positive power, wherein said first subgroupconsists of a cemented lens having a divergent cemented surface, saidsecond subgroup consists of a single lens element, at least one lenssurface of said first subgroup being aspheric, and said first lens groupsatisfying the following conditions:

    -0.7<fS/fla<0.3

    1.2<fS/flb<2.3

    -1.7<fS/fc<0

where fS: the focal length of the overall system at the wide-angle end;fla: the focal length of the first subgroup; flb: the focal length ofthe second subgroup; fc: the focal length of the cemented surface in thefirst lens group and fc=rc/(Nr-Nf); andwhere rc: the radius of curvatureof the cemented surface; and Nr: the refractive index at the d-line ofthe lens element on the image side of the cemented surface; and Nf: therefractive index at the d-line of the lens element on the object side ofthe cemented surface.
 9. A zoom lens system according to claim 8,wherein said first subgroup includes a negative lens element and apositive lens element cemented together, said first lens groupsatisfying the following conditions:

    -0.3<fS/fla<0.3

    -0.7<fS/fc<0

where N1: the refractive index of the first lens element at the d-line;and N2: the refractive index of the second lens element at the d-line.10. A zoom lens system, comprising:in order from the subject side, afirst lens group having a positive power; and a second lens group havinga negative power, the distance between said first lens group and saidsecond lens group being varied to perform zooming, said first lens groupconsists of, in order from the object side, a first subgroup and asecond subgroup having a positive power, wherein said first subgroupconsisting of a cemented lens having at least one aspheric surface, saidsecond subgroup consists of a cemented lens having a divergent cementedsurface, and said first lens group satisfying the following conditions:

    -0.7<fS/fla<0.3

    1.2<fS/flb<2.3

    -1.7<fs/fcb<-0.3

where fS: the focal length of the overall system at the wide-angle end;fla: the focal length of the first subgroup; flb: the focal length ofthe second subgroup; fcb: the focal length of the cemented surface inthe second subgroup and fcb=rcb/(N4-N3), where rcb is the radius ofcurvature of the cemented surface of the second subgroup; andwhere rc:the radius of curvature of the cemented surface; and N3: the refractiveindex at the d-line of the lens element on the image side of thecemented surface; and N4: the refractive index at the d-line of the lenselement on the object side of the cemented surface.
 11. A zoom lenssystem according to claim 10, wherein said first subgroup includes anegative lens element and a positive lens element cemented together,said first lens group satisfying the following conditions:

    -0.3<fS/fla<0.3

    -1.7<fS/fcb<-0.3.


12. A zoom lens system, comprising:in order from the subject side, afirst lens group having a positive power; and a second lens group havinga negative power, the distance between said first lens group and saidsecond lens group being varied to perform zooming, said first lens groupcomprising, in order from the object side, a first subgroup and a secondsubgroup having a positive power, said first subgroup comprises a singlelens element having at least one aspheric surface, said second subgroupcomprising a cemented lens which includes a positive first lens and anegative lens element, and which has a positive power and a divergentcemented surface, said first lens group satisfying the followingconditions: -0. 7<fS/fla<0

    1.2<fS/flb<2.3

    -1.7<fS/fcb<-0.3

where fS: the focal length of the overall system at the wide-angle end;fla: the focal length of the first subgroup; flb: the focal length ofthe second subgroup; fcb: the focal length of the cemented surface inthe second subgroup and fcb=rcb/(N3-N2); andwhere rcb: the radius ofcurvature of the cemented surface of the second subgroup; N2: therefractive index of the second lens element at the d-line; and N3: therefractive index of the third lens at the d-line; fc: the focal lengthof the cemented surface in the first lens group and fc=rc/(Nr-Nf);andwhere rc: the radius of curvature of the cemented surface; and Nr:the refractive index at the d-line of the lens element on the image sideof the cemented surface; and Nf: the refractive index at the d-line ofthe lens element on the object side of the cemented surface.
 13. A zoomlens system according to claim 12, wherein the negative lens element ofsaid cemented lens comprises an aspheric lens and said cemented lenssatisfies the following condition:

    vb<34

where vb: the Abbe number of the negative lens element cemented lens ofthe second subgroup.
 14. A zoom lens system, comprising:in order fromthe subject side, a first lens group having a positive power; and asecond lens group having a negative power, the distance between saidfirst lens group and said second lens group being varied to performzooming, said first lens group comprising, in order from the objectside, a first subgroup and a second subgroup having a positive power,wherein said first subgroup includes a negative lens element and apositive lens element cemented together and has at least one asphericsurface, said second subgroup having a single lens element having alarge positive power, said first lens group satisfying the followingconditions:

    -0.7<fS/fla<0.3

    1.2<fS/flb<2.3

    -0.7<fS/fc<0

where fS: the focal length of the overall system at the wide-angle end;fla: the focal length of the first subgroup; flb: the focal length ofthe second subgroup; fc: the focal length of the cemented surface in thefirst subgroup and fc=rc/(Nr-Nf); andwhere rc: the radius of curvatureof the cemented surface; and Nr: the refractive index at the d-line ofthe lens element on the image side of the cemented surface; and Nf: therefractive index at the d-line of the lens element on the object side ofthe cemented surface.
 15. A zoom lens system, comprising:in order fromthe subject side, a first lens group having a positive power; and asecond lens group having a negative power, the distance between saidfirst lens group and said second lens group being varied to performzooming, said first lens group comprising, in order from the objectside, a first subgroup and a second subgroup having a positive power,wherein said first subgroup includes a negative lens element and apositive lens element cemented together and has at least one asphericsurface, said second subgroup being a cemented lens having a divergentcemented surface, said first lens group satisfying the followingconditions: -0. 3<fS/fla<0.3

    1.2<fS/flb<2.3

    -1.7<fS/fc<-0.3

where fS: the focal length of the overall system at the wide-angle end;fla: the focal length of the first subgroup; flb: the focal length ofthe second subgroup; fc: the focal length of the cemented surface in thefirst lens group and fc=rc/(Nr-Nf); andwhere rc: the radius of curvatureof the cemented surface; and Nr: the refractive index at the d-line ofthe lens element on the image side of the cemented surface; and Nf: therefractive index at the d-line of the lens element on the object side ofthe cemented surface.